As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which exhibit high energy density and voltage and have long cycle lifespan and low self-discharge rate, are commercially available and widely used.
Among components of lithium secondary batteries, cathode active materials play a critical role in determining battery capacity and performance.
As cathode active materials, lithium cobalt oxides (e.g., LiCoO2) having relatively excellent physical properties, such as excellent cycle characteristics and the like, are mainly used. However, cobalt used in LiCoO2 is a rare metal and supply of cobalt is unstable because reserves and production thereof are limited. In addition, LiCoO2 is expensive due to unstable supply of cobalt and increasing demand for lithium secondary batteries.
Under these circumstances, research on cathode active materials that can replace LiCoO2 is continuously underway and, as representative alternative materials, lithium composite transition metal oxides including at least two transition metals selected from among nickel (Ni), manganese (Mn), and cobalt (Co) may be used.
Such lithium composite transition metal oxides exhibit excellent electrochemical properties through combination of high capacity of a lithium nickel oxide (e.g., LiNiO2), thermal stability and low price of Mn in a lithium manganese oxide (e.g., LiMnO2) having a layered structure, and stable electrochemical properties of LiCoO2. However, it is not easy for such lithium composite transition metal oxides to be synthesized by simple solid-phase reaction.
Thus, such lithium composite transition metal oxides are prepared by separately preparing a composite transition metal precursor including at least two transition metals selected from among Ni, Mn, and Co by a sol-gel method, a hydrothermal method, spray pyrolysis, co-precipitation, or the like, mixing the composite transition metal precursor with a lithium precursor, and calcining the resulting mixture at high temperature.
A composite transition metal precursor is generally prepared by co-precipitation in consideration of cost, productivity, and the like.
Conventionally, in a case of preparation of a composite transition metal precursor by co-precipitation, to prepare a lithium composite transition metal oxide as a cathode active material having high discharge capacity, excellent lifespan characteristics, excellent rate characteristics, and the like, preparation of the composite transition metal precursor is performed focusing on optimization of particle shapes such as spherizing or the like. In this regard, structural properties in addition to spherizing of composite transition metal precursors are very important.
However, conventional composite transition metal precursor particles prepared by co-precipitation exhibit wide particle size distribution, have non-uniform shape, and contain a large amount of impurities.
In addition, conventional composite transition metal precursor particles prepared by co-precipitation have a minimum average diameter of 6 μm to 10 μm.